The long term goal of this project is to determine the mechanism of recombination of virus DNA with the host chromosome (site-specific recombination). Recombination between the bacteriophage lambda and its host escherichia coli involves specialized regions in each DNA called attachment (att) sites. We have isolated and sequenced three mutations that change the specificity of site-specific recombination. The mutations are located in a 7 base pair segment of the att site called the overlap region. The properties of these mutations strengthen our conclusion that recombination between att sites requires a direct interaction between indentical nucleotides of the two participating overlap regions. We have isolated a specialized transducing phage that carries a host gene whose product is required for site-specific recombination. Use of this phage has enabled us to construct a restriction map of the gene. We found that a mutation that increases the efficiency of site-specific recombination is located within the phage int gene. We have continued our characterization of endonuclease VII -- an enzyme that cleaves an intermediate in genetic recombination known as a Holliday structure. The products of enzyme action are two unbranched double-stranded DNA molecules each with a single nick. The nicks can be sealed with DNA ligase.